The present invention relates generally to a multilayer air filter pack and more particularly to a multilayer air filter pack comprising at least three adjacent layers of fibrous material with each layer having a permanent electric charge.
The term "fibrous electret" is used hereinafter to describe a fibrous web of material which carries a permanent charge. Such a material is known and when stored under typical conditions can retain a useful charge for many years. Under accelerated testing, such as storage in a room-temperature 100-percent relative humidity environment, the charge on this type of fibrous material may have a half-life ranging from one week to six months or a year. With such a persistence of charge, fibers and fibrous webs of material of this type can be properly termed electrets. U.S. Pat. No. 4,215,682 discloses an example of one such fibrous electret material that may be employed in the layers of the filter pack disclosed herein.
In German Patent 29 41 094, a filter element is disclosed that comprises the combination of an uncharged filter material acting as a prefilter and a fibrous electret filter material acting as a postfilter. According to Example 3 of this patent, the filter pack can be constructed with multiple layers and with fiber layers of the same design, but in order to keep coarser particles away from the fibrous electret filter material, the filter pack is always used in combination with an uncharged prefilter. Such a filter element does not meet the frequently occurring requirement of reliably filtering small to very small particles (i.e., from 10 to 0.05 um) and bacteria, when the filter element is loaded by a considerable quantity of particles, a majority of which, or all of which, are in this size range.
In the German Patent discussed above a medium-coarse uncharged prefilter allows small particles to pass therethrough unimpeded. These particles immediately load the adjoining fibrous electret filters to their full extend at the outset of the filtering process. If these fibrous electret filters have, as is disclosed in the German patent as being preferred, a low air resistance, they act almost exclusively as an electrostatic filter and become rapidly ineffective as very-fine filters due to neutralization of their charge. This allows a very large number of impinging small particles to pass through the filters, which now effectively act solely as pure coarse filters. The degree of filtration of the small particles rapidly drops by up to a factor of 10 of the initial value.
In order to avoid this detrimental effect, the fibrous electret layers in filter elements constructed according to the German patent could be made so dense that the electrostatic effect cooperates with the mechanical separation effect. However, this would lead to an intolerably high pressure drop and such a filter element would be rapidly clogged-up with particles if the filter elements were subjected to a large quantity of particles. If the uncharged prefilter were made denser, the air resistance of the entire air filter element likewise would be increased in an unfavorable manner.
The present invention is directed to the problem of providing a fibrous electret filter pack that can withstand loading with particles, a majority of which, or all of which, are in the size range of 10 to 0.05 um in diameter and still obtain long service life, the best possible filter performance and low air resistance, without exhibiting the above-mentioned disadvantages. In order to achieve this, it is important that as many of the small particles as possible are intercepted on the upstream or input side of the filter pack.